1. Field of the Invention
The present invention relates generally to volume holograms and more specifically to volume holograms for use in optical data storage systems.
2. Description of the Prior Art
Holograms are recordings of light intensity patterns created by the interference of two beams of mutually coherent light (the two beams are usually obtained by splitting a single laser beam). There are two major categories of holograms: transmissive and reflective. These two categories are further divided into two physical types of holograms: surface relief holograms and volume holograms. Surface relief holograms can be recorded using photolithographic processes. The interference pattern is recorded as a periodic variation in thickness of the material while the refractive index of the material remains fixed.
In a volume hologram, the interference pattern is recorded as a periodic variation in the refractive index of the material while the thickness of the material remains fixed. The periodic variation in refractive index creates surfaces of peak refractive index within the material. These surfaces are referred to as "Bragg surfaces." When the interference pattern is created by two plane waves or two waves with identical curvature at the hologram surface, the Bragg surfaces will be Bragg planes.
When the hologram is re-illuminated by one of the original beams at an angle that results in maximum diffraction efficiency, the internal angle of the beam relative to the Bragg planes is referred to as the "Bragg angle." The external angle of incidence at which maximum diffraction efficiency occurs is also often referred to as the Bragg angle.
Recently, surface relief holograms have been used for optical data storage applications. These holograms allow beams of light to be separated for various purposes. These surface relief hologram systems include the following Japanese applications: JP 1-13246, published Jan. 18, 1989; JP 1-55745, published Mar. 2, 1989; JP 1-55746, published Mar. 2, 1989; JP 1-86337, published Mar. 31, 1989; JP 1-86332, published Mar. 31, 1989; JP 1-146143, published Jun. 8, 1989; JP 1-53359, published Mar. 1, 1989; JP 63-25845, published Feb. 3, 1988; JP 62-219340, published Sep. 26, 1987; and JP 61-123032, published Jun. 10, 1986.
A problem with surface relief holograms is that they are not able to achieve efficient polarization separation except for a limited range of diffraction angles. Polarization separation is very important in magneto-optic systems where the data is sensed as changes in the plane of polarization of light reflected from the optical media.
Volume holograms can be used to do polarization separation over a wide range of diffraction angles. U.S. Pat. No. 4,497,534 issued Feb. 5, 1985 shows the use of a volume hologram for polarization separation. However, volume holograms are limited in thickness to approximately 25 microns and this results in beam separations at relatively large angles of 30-45 degrees. Smaller beam angle separations are preferred in optical storage systems so that the optical sensors may be located close together and preferably in a single package.
What is needed is a volume hologram system which can achieve beam separation at desired angles.